1. Field of the Invention
The present invention relates to a process for safeguarding transmission of digital signals, in particular items of data, across short waves wherein the encoded signals provided with data protection are transmitted block-wise (burst) from a transmitting station to a receiving station and, prior to the transmission of a further burst, the receiving station returns an acknowledgment signal to the transmitting station (automatic request).
2. Description of the Prior Art
Processes of the general type set forth above are described, for example, in the book by L. Wiesner entitled "Fernschreib- und Datenubertragung uber Kurzwelle", 3rd Edition, 1980, pp. 105-114, at 184-188. The burst transmission accompanied by acknowledgment thereof, in combination with data protection, permits slightly disturbed bursts to be freed of transmission errors with regard to information content at the receiving end. Moreover, the same block can be retransmitted as and when required, if it is recognized on the basis of the response in the transmitting station that the information block has arrived in a faulty condition in a non-correctable manner at the receiving end. The encoding further permits interception-resistant transmission.
This form of protective transmission is no longer sufficient, however, when deliberate interference must be dealt with. On the other hand, deliberate interference must always be expected in radio systems used for tactical purposes. Although such radio systems generally employ a considerably higher frequency range than the short wave range, in practice it is often unavoidable to use a short wave range for reasons of unfavorable land conditions or range conditions. Therefore, in such cases it is necessary to take special measures to safeguard the communications transmission in this range.
In order to achieve reliable short wave transmission by a simple technique, the data rate is limited to approximately 200 bit/s. Three bytes corresponding to 24 bits is considered the minimum quantity of information. Taking into account the additional data protection, the minimum length for the bursts which represent the data parcels amounts to &gt;200 ms. If the source of interference is aware of the frequency of the connection, the burst length is too high in consideration of forthcoming technology. In addition, the mutual, relatively long lasting phasing-in procedure between two stations connected to one another prior to the actual transmission process proves uncommonly liable to interference. If an interference source is successful in interfering with the establishment of the connection by the use of deliberate measures, communications transmission becomes fundamentally impossible.